sex differences 2

ORGANIZATIONAL / ACTIVATIONAL EFFECTS

  • Permanent alteration of the nervous system vs. transient influence of brain function

    • Prenatal actions of hormones organize the brain in a sex-typical fashion.

    • Adult actions of hormones activate brain circuits for sex-typical behaviors.

    • There is a critical sensitive period during which hormones can be altered, resulting in changes to sexual behavior.

ORGANIZATIONAL / ACTIVATIONAL EFFECTS OF HORMONES

  • Hypothalamus

    • Releases gonadotrophin releasing hormone (GnRH) into the hypophyseal portal system, a small vessel system connecting hypothalamus and anterior pituitary.

  • Anterior Pituitary

    • Releases luteinizing hormone (LH) and follicle stimulating hormone (FSH) into systemic blood circulation.

  • Gonads

    • Release sex hormones into systemic circulation:

    • Testosterone (produced by testes)

    • Estradiol and Progesterone (produced by ovaries).

ORGANIZATIONAL / ACTIVATIONAL EFFECTS

  • Organizing sex differences in the brain

    • Peak in testosterone production.

    • Continuous testosterone production:

      • E18 (embryonic stage) to birth, postnatal days (P10) to P40, and during puberty.

    • Estrus cycle involves fluctuating levels of estrogens.

SEX DIFFERENCES IN THE BRAIN: ORGANIZATIONAL EFFECTS

  • SDN POA: Sexually dimorphic nucleus of the preoptic area.

    • Both testosterone and estradiol can mediate this sex difference in rats.

    • Hormonal modification during critical developmental periods alters brain organization in female birds, but not males.

    • Male brain organization is primarily dominated by sex chromosomes rather than hormones, indicating early life testosterone is not necessary.

LINEAR-UNITARY MODEL

  • Examines the relationship between genetic, gonadal, and hormonal sex.

    • Questions accuracy of the model, considering whether sex differences arise from chromosomal or hormonal origins.

SEXUAL DIFFERENTIATION

  • Exploration of Male/Female differences

    • Defines what constitutes a male and female.

SEX DIFFERENCES IN MORPHOLOGY AND BEHAVIOR

  • Examination of criteria defining male and female.

    • Anisogamy leading to sexual conflict is highlighted.

COMPETITION FOR MATES

  • Mate competition should theoretically be equal, although it often is not.

    • Isogamy (single gamete type) is rare (observed in algae and fungi).

    • Anisogamy contributes to complexities seen in sexually reproducing animals.

  • Effects of parental investment lead to intense competition and mate choice dynamics.

WHAT DEFINES THE SEXES?

  • Distinct categories include:

    • Heterogametic sex

    • Chromosomal sex

    • Gonadal sex

    • Hormonal sex

    • Morphological sex

    • Behavioral sex

CHROMOSOMAL SEX IN BIRDS

  • Females: Heterogametic (ZW)

  • Males: Homogametic (ZZ)

  • Males are considered the “default” sex; females arise from active hormonal processes.

SEXUAL DIMORPHISM

  • Not all species exhibit sexual dimorphism.

THE FOUR PROCESSES OF SEXUAL DIFFERENTIATION

  • Masculinization: Active process.

  • Feminization: Default process.

  • Defeminization: Active suppression of female traits.

  • Demasculinization: Active suppression of male traits.

"Y" MAKES YOU MALE

  • The primary gene responsible for testicular differentiation is on the Y chromosome in mammals.

    • Sry gene: Found on Y chromosome, responsible for testes development. SRY = sex-determining region on Y chromosome.

    • SRY gene is both necessary and sufficient for male sex determination, triggering events that lead to testes formation.

TESTIS-DETERMINING FACTOR

  • TDF (Testis-Determining Factor): Protein encoded by SRY gene.

    • If SRY gene is expressed, germinal ridge becomes testis; if not, ovary forms.

    • Partial expression results in incomplete gonadal differentiation.

  • Bipotential gonads show responses to gene expressions.

DEVELOPMENT OF MALE INTERNAL AND EXTERNAL SEXUAL ORGANS

  • The differentiation of gonadal cells into Sertoli cells and Leydig cells is guided by the Sox9 gene, activated by SRY protein.

    • Sertoli cells secrete Anti-Müllerian hormone (AMH) and facilitate Leydig cell development.

    • Leydig cells secrete testosterone; both testosterone and AMH are crucial for male organ development

    • Wolffian ducts develop into seminal vesicles and vas deferens.

    • Regression of Müllerian ducts into female reproductive structures occurs due to AMH.

ANOMALIES OF SEXUAL DIFFERENTIATION

  • Female Anomalies

    • Turner syndrome: A chromosomal condition with an absence of one X chromosome.

    • Phenotypically female but may require hormone therapy for puberty.

    • Short stature, webbed neck, and abnormal gonadal development.

    • Congenital Adrenal Hyperplasia (CAH): Overproduction of adrenal androgens due to improper metabolism causing hormonal imbalance.

    • Treatment involves surgery and lifelong corticosteroid therapy.

  • Male Anomalies

    • Androgen Insensitivity (Testicular Feminization Mutation): Results in individuals with XY chromosomes presenting as female due to non-functional androgen receptors.

    • Born looking female with a short-blind pouch vagina; sterile, absent uterus, and secondary sex characteristics like menstruation.

    • 5 Alpha Reductase Deficiency: Mutation affecting the conversion of testosterone to dihydrotestosterone leading to ambiguous genitalia.

    • Typically assigned female at birth but develops male characteristics at puberty due to androgen secretion.

    • Trisomic Genetic Anomalies

    • Klinefelter Syndrome (XXY): Sexed as males at birth, typically sterile with associated learning disabilities.

    • XYY Syndrome: Sexed as males, often sterile and may exhibit increased height and cognitive challenges.